The present invention relates to an energization control apparatus for a glow plug in a diesel engine.
In a conventional diesel engine, a glow plug is mounted on each combustion chamber of a cylinder head to facilitate starting of the engine during a cold period. When the engine is to be started, the glow plug is energized and heated to increase a compressed air temperature of the cylinder head to assure starting of the engine. In general, in such a glow plug, power is supplied to the glow plug through an energization control apparatus which is operated simultaneously with ON operation of a key switch. A high power is supplied to the glow plug to rapidly heat the plug in the initial period. After rapid heating is completed, a low power is supplied to the glow plug to stabilize heating. In general, stable heating of the glow plug is called after glow. Warming in the combustion chamber is accelerated by the after glow, and knocking of the diesel engine is prevented. In addition, noise and white smoke are also prevented, and exhaust of an HC component is suppressed.
In a conventional energization control apparatus for a glow plug, a power transistor is connected in series with a glow plug power line. The power transistor is turned on/off to control the power supplied to the glow plug. FIG. 4 shows an example of such a conventional energization control apparatus. One terminal of a sensing resistor Ra is connected to the emitter of an npn transistor 1. Glow plugs GP1 to GP4 as glow plugs 2 are connected between the other terminal of the sensing resistor Ra and ground. In other words, a timing for applying a base voltage VB to the npn transistor 1 is controlled by a controller 3, and the ON/OFF time of the npn transistor 1 is controlled. As a result, the amount of power to be supplied to the glow plugs 2 is controlled.
A typical example of the energization control apparatus employing the above scheme is disclosed in Japanese Patent Laid-Open No. 61-25971.
In the conventional energization control apparatus for a glow plug, however, the base voltage VB has substantially the same amplitude as that of the battery voltage. When an emitter voltage VE of the transistor 1 is increased upon an increase in resistance caused by a temperature rise of the glow plugs 2, a difference (VB-VE) between the base and emitter voltages VB and BE is decreased. The transistor 1 which is supposed to be used in a saturation region is undesirably used in an active region. Therefore, energization control of the glow plug 2 cannot be stably performed.
In order to always use the npn transistor 1 in the saturation region against resistance variations (load variations) in the glow plugs 2, another conventional energization control apparatus (e.g., Japanese Utility Model Publication No. 60-34786) is known. In this apparatus, the emitter of an npn transistor 1 is grounded, and glow plugs 4 are connected to the collector of the npn transistor 1. With this circuit arrangement, bipolar glow plugs 4 must be used in place of the unipolar glow plugs 2 (glow plugs commonly grounded in the housing) shown in FIG. 4. The number of wiring lines is increased, and working inefficiency and wiring errors occur, thus degrading reliability of the energization control apparatus.
As shown in FIG. 6, still another conventional energization control apparatus is proposed (e.g., Japanese Patent Laid-Open Nos. 59-119070 and 59-122782) wherein a pnp transistor 5 is used in place of the npn transistor 1 shown in FIG. 4, and the glow plugs 2 are connected between ground and the collector of the pnp transistor 5. However, pnp transistors for driving glow plugs 2 with a large current are hard to get on the market. For this reason, easily accessible pnp transistors having a relatively small capacity are connected in parallel with each other to satisfy the need for high-power driving of the glow plugs. In this case, the pnp transistors must have identical characteristics, and therefore, the energization control apparatus becomes expensive as a whole.